This invention relates to coded, carrier-modulated data communication systems in which dependencies are introduced between successive transmitted signal points and the receiver delays its final decision on each received signal in order to take into account subsequently received signals.
In typical such coded systems (for example the systems described in Csajka et al., U.S. Pat. No. 4,077,021, and Ungerboeck, "Channel Coding with Multilevel/Phase Signals", IEEE Transactions on Information Theory, Vol. IT-28, No. 1, January, 1982) the receiver uses a Viterbi algorithm trellis decoder (of the kind described in Forney, "The Viterbi Algorithm", Proceedings of the IEEE, 61(3):268 (March 1973), incorporated herein by reference) to decode a sequence of received signals into the sequence of signal points that is closest to the sequence of received signals in the sense of the algebraic sum of squared Euclidean distances. Final decisions from the decoder are delayed for a sufficient number of signaling intervals to assure to an acceptably high probability that the sequence of which signal points were sent will be correctly decided.
Typical receivers also include equalizers to reduce the effects of intersymbol interference introduced by the channel, as described in Qureshi, "Adaptive Equilization", IEEE Communications Magazine, March, 1982, incorporated herein by reference. For channels with severe amplitude distortion, the typical linear transversal equalizer used in quadrature amplitude modulation (QAM) systems enhances noise and correlates the noise samples (i.e., the noise components in the respective received signals) in successive intervals. In coded systems using the conventional Viterbi algorithm decoder, such noise correlation can significantly increase the probability of making decision errors.
In conventional so-called uncoded systems, a decision feedback type equalizer (DFE) can be substituted for the linear equalizer to perform equalization with less noise enhancement, and without correlating noise samples. Generally, a DFE multiplies previous decisions by feedback coefficients and sums the products to produce a valve to be applied to the demodulated, partly equalized, received signal to correct for the anticipated intersymbol interference (due to previous signal points) in the currently received signal. DFEs are described in the Qureshi article cited above, in C. A. Belfiore and J. H. Park, Jr., "Decision Feedback Equalization", Proceedings of the IEEE, August, 1979, and in D. D. Falconer, "Application of Passband Decision Feedback Equalization in Two-Dimensional Data Communication Systems", IEEE Transactions on Communications, October, 1976, all incorporated herein by reference.
An alternative form of DFE (called a noise predictor) can be used to predict and compensate for the noise component in the received signal at the feedforward linear equalizer output, as described in the Belfiore and Park article. The noise predictor output is a weighted sum of past error signals (each based on a comparison of a past received signal with the corresponding decision), where the weighting coefficients are selected to minimize the average power of the residual noise signals after prediction by removing the correlation which exists between successive error signals before prediction. Unlike the conventional-form DFEs, in noise predictors the coefficients of the linear (or forward) equalizer are independent of the predictor (or feedback) coefficients and the forward equalizer coefficients can be updated to minimize the mean square error before prediction as in a conventional linear equalizer.
In using decision-feedback techniques with conventional uncoded systems, the decisions required for feedback are available without delay. In coded systems, however, reliable decisions are available only after some delay; and tentative decisions, which may be available with little or no delay, are generally unreliable and would result in excessive error propagation which would eliminate the performance advantage of decision-feedback equalization.
Two techniques for applying decision feedback techniques to Viterbi algorithm decoding are disclosed in Qureshi, U.S. Pat. No. 4,631,735 issued Dec. 23, 1986, and in Eyuboglu, U.S. Pat. No. 4,713,829 issued Dec. 15, 1987, both assigned to the same assignee as this application, and incorporated herein by reference. In the Qureshi application, the receiver associates a separate decision-feedback noise predictor circuit with each possible state of the Viterbi decoder. In the Eyuboglu application, the receiver associates decision-feedback noise predictor circuits with so-called decision subsets. In both cases, the predictors use appropriate tentative decisions from the previous signaling interval for feedback. Both the Qureshi and Eyuboglu schemes are capable of reducing the effects of channel distortion in coded systems relative to conventional linear equalization.
In data communication systems of the kind that have error-control decoders so-called interleaving schemes have been used to remove the effects of channel burst noise as described, for example by A. Viterbi and J. Omura in "Principles of Digital Communication and Coding", McGraw Hill Book Company, 1979, incorporated herein by reference. An interleaver rearranges the order of the symbols in an input sequence. A corresponding deinterleaver performs the inverse operation to recover the original sequence, but with the noise samples spread out.
One class of interleavers, called periodic interleavers, is disclosed in Forney, U.S. Pat. No. 3,652,998, issued March 1972, assigned to the same assignee as this application, and incorporated herein by reference.